Input: ads7846 - make sure we do not change platform data

Let's declare platform data a const pointer so that we don't accitentally
change it. Also fetch it with dev_get_platdata().

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Input: ads7846 - add device tree bindings

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>